1. Field of the Invention
The present invention relates to a brake band for use in an automatic transmission or the like in which initial frictional characteristic is improved and durability is provided, and producing method thereof.
2. Description of the Related Art
In a conventional wet frictional engagement plate, a frictional material is fixed on a surface of a core metal. The frictional material consists of: a fiber base such as natural f pulp fibers, organic synthetic fibers, or the like; a filler such as diatom earth or the like; a friction adjustment agent such as cashew resin or the like; and thermosetting resin. Therefore, the frictional surface is slightly uneven because of interweave of the fibers so that there is such a tendency that the friction coefficient is small in an initial stage of use because of presence of the thermosetting resin attached on the fiber surface, and the friction coefficient becomes larger because the frictional surface gets used with passage of time of use.
For example, in FIG. 1, the ordinate and abscissa represent the friction coefficient and the number of use (number of cycles) respectively and a dotted line B shows a change of the friction coefficient of the conventional frictional plate. It is found from the drawing that the friction coefficient is small in the initial stage of use and becomes large gradually with the passage of time of use and that the value of the friction coefficient is not stabilized until about 100 cycles.
In the case of a vehicle having an automatic transmission, therefore, there has been such a disadvantage that the running time of the transmission is shortened because the transmission is returned when the sense of driving changes due to to a change in the friction coefficient from an initial state.
Having the foregoing disadvantage, the conventional frictional material has such a problem that the friction coefficient changes between the beginning of use and after use for a certain period of time to thereby cause a large change in the performance of transmission when the frictional material is used as a brake band of an automatic transmission.
Then, in order to eliminate the disadvantage, there have been proposed various methods. For example, another slidable member, which is slidable with respect to a frictional plate, is compressedly contacted to the frictional plate, and is slid for a predetermined time to increase the temperature of the frictional surface of the frictional plate so as to harden an unhardened resin component on the frictional surface to thereby stabilize the friction coefficient.
Although the friction coefficient can be stabilized in this method, it requires a certain amount of time for running-in in oil with a partner member in order to obtain predetermined performance, and there arises a problem in workability which thereby causes an increase of the cost.
On the other hand, a surface portion of a frictional material is irradiated with laser light to burn out or partly thermally decompose an organic binder to thereby convert at least a part of the organic binder to carbon (i.e, to carbonizer) so as to remove excessive organic binder to thereby prevent initial reduction of the friction coefficient.
Further, there has been proposed such a method that a hot plate heated to 600.degree. C. is pressed on a frictional material to carbonize a binder in the frictional material.
In the foregoing means, although a frictional material is heated to a temperature not lower than the temperature of decomposition of resin to carbonize the resin so that the friction coefficient is stabilized, there has been such a significant problem that the strength of the frictional material is reduced or the durability thereof is reduced by abrasion of a surface carbonized-layer or the like.